A standard twelve lead electrocardiogram (ECG) is obtained using an arrangement of ten electrodes placed at specific locations on the patient's body. It is understood within the present invention that an electrode is a solid electrical conductor placed on the patient's body through which an electric current enters or leaves the patient's body. It is further understood that within the present invention a lead is an electrical slice or view of the patient's heart with respect to the electrocardiographic signal obtained at one or more electrocardiographic electrodes. In a standard twelve lead ECG, four of the ten electrodes are placed on the patient's limbs and the other six electrodes are placed over the patient's heart and across the left side of the patient's chest. Typically, electrocardiographic signals are obtained from each of the ten electrodes and processed to generate the twelve leads of the ECG. The twelve leads are further split into two groups of six ECG leads: the frontal plane leads (I, II, III, aVR, aVL, and aVF) and the horizontal plane leads (V1, V2, V3, V4, V5, V6).
While the standard twelve lead ECG is very useful in obtaining valuable information regarding the patient's heart and cardiac function, it is sometimes undesirable to attach ten electrodes to the patient to obtain the data necessary for a twelve lead ECG. Systems have been developed, such as that disclosed in U.S. Pat. No. 4,850,370, to provide a solution to limit the number of electrodes that must be attached to the patient to obtain all of the desired electrocardiographic information. The '370 patent discloses a novel arrangement of four electrodes on the patient's chest to obtain cardiographic data. However, the system shown in the '370 patent is limited because the electrocardiographic signals produced from this electrode arrangement are inherently different than a standard twelve lead ECG due to the different electrode placements. Much of a cardiologist's or clinician's ability to accurately interpret ECG data is based upon their familiarity with the signals produced at each of the ECG leads. The differences in the electrocardiographic signals produced from an implementation of the '370 patent as compared to a standard twelve lead system requires that the cardiologist or clinician learn an entirely new set of ECG waveforms.
To address these problems, systems such as disclosed in U.S. Pat. No. 6,636,761 to Brodnick have been developed that use some of the ten electrode placements required for a standard twelve lead ECG to monitor some of the twelve leads and to mathematically derive the other unmonitored leads. In an embodiment of the '761 patent, only the four limb electrodes and two precordial (chest) electrodes are utilized to obtain patient information, but in other embodiments more or less of the ten electrode placements may be used.
While the implementation of the system described in the '761 patent provides ECG data to clinicians in a form that they are used to interpreting while at the same time reducing the number of electrodes to be attached to the patient, automated ECG interpretation systems have difficulty interpreting results from systems that include derived ECG leads. While the derived ECG leads are typically accurate in terms of the signal interval and duration, there may be errors in the amplitude of some of the derived ECG leads. The amplitude difference in the derived ECG as compared to the actual lead if monitored can be enough to alter the automated interpretation of the ECG if the interpretation is based heavily upon the amplitude of the derived ECG leads.
A specific example of this potential error is the detection of anterior myocardial infarction (MI), especially acute MI. The automated detection of anterior MI utilizes precordial leads V2, V3 and V4, which in a preferred embodiment of the '761 patent are replaced by the monitoring of leads V1 and V5 coupled with the derivation of V2, V3, and V4. Thus, the automated detection of anterior MI relies upon only derived leads. Therefore, any error in the amplitude measure for the derived leads can alter the automated interpretation of the ECG with respect to the detection of anterior MI. The use of the standard twelve lead ECG interpretation algorithms with the derived ECG leads is estimated to account for a reduction of approximately 10% to 15% in the accuracy of the automated detection of anterior MI.
Therefore, it is desirable in the field of ECG collection and automated ECG interpretation to develop a method and apparatus for the improved automated interpretation of ECG leads that are collected from fewer than ten ECG electrodes.